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          <h1 class="post-title" itemprop="name headline">大并发服务器框架设计</h1>
        

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        <p>简单谈一谈<code>大并发服务器框架</code>设计的基本思路<br>基本的服务器框架都是<code>C/S结构</code>的，请求和相应流程是这样的：<br><img src="ioservertype/1.png" alt="1"><br>这样的框架存在一个很严重的问题，当客户端大并发请求到来，服务器需要进行大量的数据库操作，假设数据库最大连接数为1000个，此时有10000个请求访问应用服务器，那么应用服务器只能处理1000个请求，剩下99000个等待1000个请求处理好后再进行访问数据库处理。可以在应用服务器和数据库服务器中间增加中间层DAL，DAL采用缓冲队列和连接池设计。<br><a id="more"></a><br><img src="ioservertype/2.png" alt="2"><br>DAL设计缓冲队列，存储等待的请求，并且DAL中设计数据库连接池，当数据库连接池中有空闲连接，那么从缓冲队列中取出一个请求处理，以此类推。这种做法有效的降低了服务器的压力，但是没有提高处理速度，仅仅保证了请求被缓存，处理效率仍受限于数据库的并发数。那么可以再增加一层缓存，将常用的数据加载如缓存，有请求到来时，应用服务器先从缓存中获取数据，如果缓存中有数据，那么不需要访问数据库，如果缓存中没有，在访问数据库取出数据，并更新缓存。<br><img src="ioservertype/3.png" alt="3"><br>缓存如何同步？</p>
<p>有两种手段：</p>
<p>第一种方法： <code>缓存是具有时效的，在一定时间过后会超时timeout，如果缓存失效，那么重新去数据库查询，查询后更新缓存，这种方法不是实时的，实时性比较差。</code></p>
<p>第二种方法：<code>当有请求修改数据时，更新缓存，并且将要修改的数据投入DAL层，当数据库有空闲连接时，再持久化存盘。</code></p>
<p>缓存的不足之处：</p>
<p>当缓存足够多时，需要将不活跃缓存数据换出内存，叫做缓存换页。缓存换出算法和操作系统换页算法类似，<code>FIFO</code>，<code>LRU（least recently used）</code>，<code>LFU（least  frequently used）</code>等。实际缓存的实现不需要自己去实现，有很多开源技术，nosql技术就是非关系型数据库的意思。非关系型数据库如redis，memcatched等。缓存可以跟应用服务器部署在同一台机器上，也可以部署在单独机器上。我推荐将缓存服务器部署在单独机器上，假设有两台应用服务器，如果将缓存部署在不同的应用服务器上，那么不同的应用服务器很难访问彼此的缓存，非常不方便。将缓存部署在单独服务器上，各个应用服务器都能访问该缓存服务器。<br><img src="ioservertype/4.png" alt="4"><br>如果有大量的业务请求到来，虽然设计了多个应用服务器，也架设了缓存服务器，完善了中间层的缓冲队列和数据库连接池，但是数据库服务器仍然会出现瓶颈。比如当有大量复杂的写操作数据库，很多读数据库的操作就被阻塞了，为解决这个问题可将数据库实现读写分离。由于数据库读操作会比写操作多，那么可以对数据库执行负载均衡。主流数据库都有replication机制，采用replication机制可以实现负载均衡。中间层的写数据库操作投递到master数据库中，读操作从slave数据库中读取，当master数据库中数据被修改后，数据库采用replication机制将数据同步给slave服务器。<br><img src="ioservertype/5.png" alt="5"><br>同样的道理，应用服务器也可以实现负载均衡，架设多个应用服务器，不同的请求分配给不同的应用服务器。可单独设计一个任务服务器监控各个应用服务器的负载情况，合理的分配任务给各个应用服务器。这种方式是任务服务器主动地分配任务给应用服务器，应用服务器被动的接受任务，这种方式在任务请求类型相近的情况下，分配方式非常合理。但是假设应用服务器A接受了3个任务，应用服务器B接受了5个任务，按照负载均衡的权重法或最小连接法，肯定会分配给A任务，但是如果这3个任务都是复杂的写操作，而B的5个任务都是简单的读操作，那么这就存在分配的不合理性，如何解决这个问题呢？<br><img src="ioservertype/6.png" alt="6"><br>可以换一种思路去解决这个问题，让应用服务器主动去请求任务服务器，主动获取任务处理，如果应用服务器处于忙碌状态就不需要请求新的任务，空闲的应用服务器会去请求任务服务器中的任务，这是最合理的负载均衡。如果所有应用服务器都处于忙碌状态，那么任务服务器将任务缓存至自己的任务队列，当应用服务器空闲时会来取任务。考虑这样一个问题，如果任务服务器出现故障怎么办？任务服务器需要有多台，并且实现failover机制，多台任务服务器之间实现心跳，如果检测不到对方心跳，则使自己成为主任务服务器。<br><img src="ioservertype/7.png" alt="7"><br>到目前为止，这个框架可以适用于大部分服务器逻辑。为保证数据库的响应速度和处理效率，可以对数据库进行分区。</p>
<p>数据库分区有两种形式(分库、分表)</p>
<p>分库：数据库可以按照一定的逻辑把表分散到不同的数据库。这叫做垂直分区，就是所每个库的表不同，功能不同。这样做不常见，因为很大情况下，数据库中各个表是关联的，</p>
<p>如果将不同的表分配到不同的数据库中，会存在很多不便。</p>
<p>分表：将一个表的不同数据分配到各个数据库，这样每个数据库的表结构是一样的，只是存储的用户数据不同而已，叫做水平分区。分表的方式很常见，如果数据库的压力增加，</p>
<p>我们就采取分表的方式减少数据库的压力。</p>
<p>另外服务器开发的几个性能杀手：</p>
<p>1 数据拷贝，数据从内核态copy到用户态，或者在用户态之间copy会造成性能损失，尽量采用缓存的方式解决。</p>
<p>2 环境切换 ，多线程上下文切换造成开销。如果服务器是单核的，那么采用状态机方式单线程效果最佳。如果是多核的，</p>
<p>合理采用多线程，可以提升性能。</p>
<p>3 内存分配，可以采用内存池，提前分配。</p>
<p>4 锁竞争，加锁解锁会造成一定的效率衰减。</p>

      
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  <script type="text/javascript">
    // Popup Window;
    var isfetched = false;
    var isXml = true;
    // Search DB path;
    var search_path = "search.xml";
    if (search_path.length === 0) {
      search_path = "search.xml";
    } else if (/json$/i.test(search_path)) {
      isXml = false;
    }
    var path = "/" + search_path;
    // monitor main search box;

    var onPopupClose = function (e) {
      $('.popup').hide();
      $('#local-search-input').val('');
      $('.search-result-list').remove();
      $('#no-result').remove();
      $(".local-search-pop-overlay").remove();
      $('body').css('overflow', '');
    }

    function proceedsearch() {
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay"></div>')
        .css('overflow', 'hidden');
      $('.search-popup-overlay').click(onPopupClose);
      $('.popup').toggle();
      var $localSearchInput = $('#local-search-input');
      $localSearchInput.attr("autocapitalize", "none");
      $localSearchInput.attr("autocorrect", "off");
      $localSearchInput.focus();
    }

    // search function;
    var searchFunc = function(path, search_id, content_id) {
      'use strict';

      // start loading animation
      $("body")
        .append('<div class="search-popup-overlay local-search-pop-overlay">' +
          '<div id="search-loading-icon">' +
          '<i class="fa fa-spinner fa-pulse fa-5x fa-fw"></i>' +
          '</div>' +
          '</div>')
        .css('overflow', 'hidden');
      $("#search-loading-icon").css('margin', '20% auto 0 auto').css('text-align', 'center');

      $.ajax({
        url: path,
        dataType: isXml ? "xml" : "json",
        async: true,
        success: function(res) {
          // get the contents from search data
          isfetched = true;
          $('.popup').detach().appendTo('.header-inner');
          var datas = isXml ? $("entry", res).map(function() {
            return {
              title: $("title", this).text(),
              content: $("content",this).text(),
              url: $("url" , this).text()
            };
          }).get() : res;
          var input = document.getElementById(search_id);
          var resultContent = document.getElementById(content_id);
          var inputEventFunction = function() {
            var searchText = input.value.trim().toLowerCase();
            var keywords = searchText.split(/[\s\-]+/);
            if (keywords.length > 1) {
              keywords.push(searchText);
            }
            var resultItems = [];
            if (searchText.length > 0) {
              // perform local searching
              datas.forEach(function(data) {
                var isMatch = false;
                var hitCount = 0;
                var searchTextCount = 0;
                var title = data.title.trim();
                var titleInLowerCase = title.toLowerCase();
                var content = data.content.trim().replace(/<[^>]+>/g,"");
                var contentInLowerCase = content.toLowerCase();
                var articleUrl = decodeURIComponent(data.url);
                var indexOfTitle = [];
                var indexOfContent = [];
                // only match articles with not empty titles
                if(title != '') {
                  keywords.forEach(function(keyword) {
                    function getIndexByWord(word, text, caseSensitive) {
                      var wordLen = word.length;
                      if (wordLen === 0) {
                        return [];
                      }
                      var startPosition = 0, position = [], index = [];
                      if (!caseSensitive) {
                        text = text.toLowerCase();
                        word = word.toLowerCase();
                      }
                      while ((position = text.indexOf(word, startPosition)) > -1) {
                        index.push({position: position, word: word});
                        startPosition = position + wordLen;
                      }
                      return index;
                    }

                    indexOfTitle = indexOfTitle.concat(getIndexByWord(keyword, titleInLowerCase, false));
                    indexOfContent = indexOfContent.concat(getIndexByWord(keyword, contentInLowerCase, false));
                  });
                  if (indexOfTitle.length > 0 || indexOfContent.length > 0) {
                    isMatch = true;
                    hitCount = indexOfTitle.length + indexOfContent.length;
                  }
                }

                // show search results

                if (isMatch) {
                  // sort index by position of keyword

                  [indexOfTitle, indexOfContent].forEach(function (index) {
                    index.sort(function (itemLeft, itemRight) {
                      if (itemRight.position !== itemLeft.position) {
                        return itemRight.position - itemLeft.position;
                      } else {
                        return itemLeft.word.length - itemRight.word.length;
                      }
                    });
                  });

                  // merge hits into slices

                  function mergeIntoSlice(text, start, end, index) {
                    var item = index[index.length - 1];
                    var position = item.position;
                    var word = item.word;
                    var hits = [];
                    var searchTextCountInSlice = 0;
                    while (position + word.length <= end && index.length != 0) {
                      if (word === searchText) {
                        searchTextCountInSlice++;
                      }
                      hits.push({position: position, length: word.length});
                      var wordEnd = position + word.length;

                      // move to next position of hit

                      index.pop();
                      while (index.length != 0) {
                        item = index[index.length - 1];
                        position = item.position;
                        word = item.word;
                        if (wordEnd > position) {
                          index.pop();
                        } else {
                          break;
                        }
                      }
                    }
                    searchTextCount += searchTextCountInSlice;
                    return {
                      hits: hits,
                      start: start,
                      end: end,
                      searchTextCount: searchTextCountInSlice
                    };
                  }

                  var slicesOfTitle = [];
                  if (indexOfTitle.length != 0) {
                    slicesOfTitle.push(mergeIntoSlice(title, 0, title.length, indexOfTitle));
                  }

                  var slicesOfContent = [];
                  while (indexOfContent.length != 0) {
                    var item = indexOfContent[indexOfContent.length - 1];
                    var position = item.position;
                    var word = item.word;
                    // cut out 100 characters
                    var start = position - 20;
                    var end = position + 80;
                    if(start < 0){
                      start = 0;
                    }
                    if (end < position + word.length) {
                      end = position + word.length;
                    }
                    if(end > content.length){
                      end = content.length;
                    }
                    slicesOfContent.push(mergeIntoSlice(content, start, end, indexOfContent));
                  }

                  // sort slices in content by search text's count and hits' count

                  slicesOfContent.sort(function (sliceLeft, sliceRight) {
                    if (sliceLeft.searchTextCount !== sliceRight.searchTextCount) {
                      return sliceRight.searchTextCount - sliceLeft.searchTextCount;
                    } else if (sliceLeft.hits.length !== sliceRight.hits.length) {
                      return sliceRight.hits.length - sliceLeft.hits.length;
                    } else {
                      return sliceLeft.start - sliceRight.start;
                    }
                  });

                  // select top N slices in content

                  var upperBound = parseInt('1');
                  if (upperBound >= 0) {
                    slicesOfContent = slicesOfContent.slice(0, upperBound);
                  }

                  // highlight title and content

                  function highlightKeyword(text, slice) {
                    var result = '';
                    var prevEnd = slice.start;
                    slice.hits.forEach(function (hit) {
                      result += text.substring(prevEnd, hit.position);
                      var end = hit.position + hit.length;
                      result += '<b class="search-keyword">' + text.substring(hit.position, end) + '</b>';
                      prevEnd = end;
                    });
                    result += text.substring(prevEnd, slice.end);
                    return result;
                  }

                  var resultItem = '';

                  if (slicesOfTitle.length != 0) {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + highlightKeyword(title, slicesOfTitle[0]) + "</a>";
                  } else {
                    resultItem += "<li><a href='" + articleUrl + "' class='search-result-title'>" + title + "</a>";
                  }

                  slicesOfContent.forEach(function (slice) {
                    resultItem += "<a href='" + articleUrl + "'>" +
                      "<p class=\"search-result\">" + highlightKeyword(content, slice) +
                      "...</p>" + "</a>";
                  });

                  resultItem += "</li>";
                  resultItems.push({
                    item: resultItem,
                    searchTextCount: searchTextCount,
                    hitCount: hitCount,
                    id: resultItems.length
                  });
                }
              })
            };
            if (keywords.length === 1 && keywords[0] === "") {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-search fa-5x" /></div>'
            } else if (resultItems.length === 0) {
              resultContent.innerHTML = '<div id="no-result"><i class="fa fa-frown-o fa-5x" /></div>'
            } else {
              resultItems.sort(function (resultLeft, resultRight) {
                if (resultLeft.searchTextCount !== resultRight.searchTextCount) {
                  return resultRight.searchTextCount - resultLeft.searchTextCount;
                } else if (resultLeft.hitCount !== resultRight.hitCount) {
                  return resultRight.hitCount - resultLeft.hitCount;
                } else {
                  return resultRight.id - resultLeft.id;
                }
              });
              var searchResultList = '<ul class=\"search-result-list\">';
              resultItems.forEach(function (result) {
                searchResultList += result.item;
              })
              searchResultList += "</ul>";
              resultContent.innerHTML = searchResultList;
            }
          }

          if ('auto' === 'auto') {
            input.addEventListener('input', inputEventFunction);
          } else {
            $('.search-icon').click(inputEventFunction);
            input.addEventListener('keypress', function (event) {
              if (event.keyCode === 13) {
                inputEventFunction();
              }
            });
          }

          // remove loading animation
          $(".local-search-pop-overlay").remove();
          $('body').css('overflow', '');

          proceedsearch();
        }
      });
    }

    // handle and trigger popup window;
    $('.popup-trigger').click(function(e) {
      e.stopPropagation();
      if (isfetched === false) {
        searchFunc(path, 'local-search-input', 'local-search-result');
      } else {
        proceedsearch();
      };
    });

    $('.popup-btn-close').click(onPopupClose);
    $('.popup').click(function(e){
      e.stopPropagation();
    });
    $(document).on('keyup', function (event) {
      var shouldDismissSearchPopup = event.which === 27 &&
        $('.search-popup').is(':visible');
      if (shouldDismissSearchPopup) {
        onPopupClose();
      }
    });
  </script>





  

  

  

  
  

  

  

  

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